Scientific Method —

The quantum elephant

Theoreticians come up with a decent memory system for quantum computers.

The development of quantum computers has been the subject of much interest in recentyears. However, progress has been severely hampered by several technical problems. For instance, quantum computing with light is easy, you just need a laser, a few commonly available optical components and a table to put them on. Unfortunately, if you want to do more than one computation you need some memory to store the data. There are ways to do this but all of them would make an engineer break into tears. Storing quantum information in atoms, ions or groups of atoms and ions is much easier. However, wafting atoms through a vacuum system to perform different computations at different quantum logic gates is no one's definition of fast. The obvious solution is to store the quantum information in matter and do the computations with light. Now this just leaves the problem of transferring the quantum state between light and matter.

Several solutions to this problem have been proposed by theorists and some of them have been demonstrated experimentally. However, none of these solutions have been adequate, since they require carefully aligned, very highly reflective mirrors. One scheme that has seen some success involves passing a pulse of light through a gas of metal atoms. The atoms themselves are sitting in a magnetic field, which forces their own spin states to either align with the field or align opposite the field (anti-aligned). When the light passes through it sets the atoms into a state where they are both aligned and anti-aligned simultaneously. This writes the quantum state of the light into the atoms. However, reading the state requires that the light pulse makes two complete passes through the gas, which means that the pulse travels a few hundred kilometers before re-entering the gas for the second pass. Worse than that, to make sure the light reads the state correctly it must be in a very special state, called squeezed light. Squeezed light is difficult to make and this is a big disadvantage.

Now the scientists have taken a close look at that particular experiment and decided that it could be improved by a few simple changes. Instead of passing the light through cell twice along the same axis, they pass the light through the cell first in one direction and then at a second time at a right angle. There is no long delay between the two passes, so the light is essentially making the two passes simultaneously. The reason this works is that the atomic spins are in a superposition that is well defined in space so the light sees a slightly different superposition in each direction. The combination of which looks exactly like a squeezed state, which is what is required for reading data with a low error rate. Furthermore, by changing the strength of the magnetic field during the light pulse and increasing the intensity of the light, the interaction between the atoms and light can be optimized, reducing the number of read errors further.

The best thing about this work (as noted by the authors) is that they just need to add eight mirrors to their existing experiment to test it. Here's hoping they can convince a graduate student to do that.

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku